U.S. patent application number 12/532426 was filed with the patent office on 2010-03-11 for elastic wave element.
This patent application is currently assigned to Panasonic Corporation. Invention is credited to Yosuke Hamaoka, Yukio Iwasaki, Hiroki Kamiguchi.
Application Number | 20100060101 12/532426 |
Document ID | / |
Family ID | 40074727 |
Filed Date | 2010-03-11 |
United States Patent
Application |
20100060101 |
Kind Code |
A1 |
Iwasaki; Yukio ; et
al. |
March 11, 2010 |
ELASTIC WAVE ELEMENT
Abstract
An elastic wave element includes a piezoelectric substrate, an
interdigital electrode provided on the piezoelectric substrate, a
silicon oxide film covering the interdigital electrode, and a
silicon nitride oxide film provided on the silicon oxide film. A
film thickness H of the silicon oxide film and a wave length
.lamda. of an elastic wave propagating through the piezoelectric
substrate satisfies a relation of H/.lamda..gtoreq.0.15. The
elastic wave element reduces fluctuation of propagation
characteristics of elastic waves, and has high reliability.
Inventors: |
Iwasaki; Yukio; (Osaka,
JP) ; Kamiguchi; Hiroki; (Osaka, JP) ;
Hamaoka; Yosuke; (Osaka, JP) |
Correspondence
Address: |
RATNERPRESTIA
P.O. BOX 980
VALLEY FORGE
PA
19482
US
|
Assignee: |
Panasonic Corporation
Osaka
JP
|
Family ID: |
40074727 |
Appl. No.: |
12/532426 |
Filed: |
May 9, 2008 |
PCT Filed: |
May 9, 2008 |
PCT NO: |
PCT/JP2008/001165 |
371 Date: |
September 22, 2009 |
Current U.S.
Class: |
310/313B ;
310/365 |
Current CPC
Class: |
H03H 9/0222
20130101 |
Class at
Publication: |
310/313.B ;
310/365 |
International
Class: |
H01L 41/08 20060101
H01L041/08; H01L 41/047 20060101 H01L041/047 |
Foreign Application Data
Date |
Code |
Application Number |
May 25, 2007 |
JP |
2007-138591 |
Mar 27, 2008 |
JP |
2008-082801 |
Claims
1. An elastic wave element comprising: a piezoelectric substrate;
an interdigital electrode provided on the piezoelectric substrate;
a silicon oxide film covering the interdigital electrode, the
silicon oxide film containing argon; and a silicon nitride oxide
film provided on the silicon oxide film, wherein a film thickness H
of the silicon oxide film and a wave length .lamda. of an elastic
wave propagating through the piezoelectric substrate satisfies a
relation of H/.lamda..gtoreq.0.15.
2. The elastic wave element according to claim 1, wherein a film
thickness of the silicon nitride-oxide film is not smaller than 3
nm and not larger than 3% of the film thickness of the silicon
oxide film.
3. The elastic wave element according to claim 1, wherein the
silicon nitride oxide film prevents the argon contained in the
silicon oxide film from being discharged to an outside of the
silicon oxide film.
4. The elastic wave element according to claim 3, wherein a film
thickness of the silicon nitride-oxide film is not smaller than 3
nm and not larger than 3% of the film thickness of the silicon
oxide film.
5. The elastic wave element according to claim 1, wherein said
elastic wave element is used in a transmission filter of a
duplexer.
Description
TECHNICAL FIELD
[0001] This invention relates to an elastic wave element including
a piezoelectric substrate.
BACKGROUND ART
[0002] An elastic wave element described in Patent Document 1
includes a piezoelectric substrate, an interdigital electrode
provided on the piezoelectric substrate, and a silicon oxide film
provided on the interdigital electrode. In order to improve
temperature characteristics of the piezoelectric substrate, the
silicon oxide film has a smaller thermal expansion coefficient than
the piezoelectric substrate, accordingly reducing a thermal
expansion of the piezoelectric substrate due to a change of
temperature.
[0003] The silicon oxide film is formed on the interdigital
electrode by a chemical vapor disposition (CVD) method or by a
sputtering method. The silicon oxide film formed by the CVD method
has a low density. The low density of the silicon oxide film does
not affect the propagation of elastic waves having frequencies of
several tens MHz having long wavelengths; however, increases a
propagation loss for elastic waves having frequencies of several
hundreds MHz to several GHz having short wavelengths. The silicon
oxide film of the elastic wave element allowing such a high
frequency elastic wave to propagate is manufactured by the
sputtering method.
[0004] The elastic wave element including the silicon oxide film
manufactured by the sputtering method may change in its propagation
characteristic of the elastic wave according to the change of
environmental factor, such as an ambient temperature change.
[0005] Patent Document 1: JP2003-209458A
SUMMARY OF THE INVENTION
[0006] An elastic wave element includes a piezoelectric substrate,
an interdigital electrode provided on the piezoelectric substrate,
a silicon oxide film covering the interdigital electrode, and a
silicon nitride oxide film provided on the silicon oxide film. A
film thickness H of the silicon oxide film and a wave length
.lamda. of an elastic wave propagating through the piezoelectric
substrate satisfies a relation of H/.lamda..gtoreq.0.15.
[0007] The elastic wave element reduces fluctuation of propagation
characteristics of elastic waves, and has high reliability.
BRIEF DESCRIPTION OF THE DRAWINGS
[0008] FIG. 1 is a cross sectional view of an elastic wave element
according to an exemplary embodiment of the invention.
[0009] FIG. 2 shows a change of composition rates of argon in a
silicon oxide film and a silicon nitride-oxide film of the elastic
wave element according to the embodiment.
REFERENCE NUMERALS
[0010] 1 Piezoelectric Substrate [0011] 2 Interdigital Electrode
[0012] 3 Silicon Oxide Film [0013] 4 Silicon Nitride Oxide Film
[0014] 22 Interdigital Electrode [0015] 1001 Elastic Wave
Element
DETAILED DESCRIPTION OF PREFERRED EMBODIMENT
[0016] FIG. 1 is a cross sectional view of elastic wave element
1001 according to an exemplary embodiment of the invention. Elastic
wave element 1001 is a surface acoustic wave element utilizing a
surface acoustic wave as an elastic wave. Elastic wave element 1001
includes piezoelectric substrate 1, interdigital electrodes 2 and
22 provided upper surface 1A of piezoelectric substrate 1, silicon
oxide film 3 made of silicon oxide provided on interdigital
electrodes 2 and 22, and silicon nitride oxide film (silicon
oxynitride film) 4 made of silicon nitride-oxide (silicon
oxynitride) provided on silicon oxide film 3. Silicon oxide film 3
covers interdigital electrodes 2 and 22. Piezoelectric substrate 1
is a rotated Y-cut X-propagation lithium niobate substrate having a
rotational angle of 5.degree.. Interdigital electrodes 2 and 22 are
made mainly of aluminum. Interdigital electrode 2 has plural
electrode fingers 102. Interdigital electrode 22 has plural
electrode fingers 122. Electrode fingers 102 and electrodes fingers
122 are arranged alternately. Elastic wave element 1001 according
to the embodiment is a filter having a center frequency of about 2
GHz. The wave length of the elastic wave (surface acoustic wave)
propagating piezoelectric substrate 1 is about 2 .mu.m which is
equal to pitches between electrode fingers 102 of interdigital
electrode 2 and pitches between electrode fingers 122 of
interdigital electrode 22. Interdigital electrodes 2 and 22 have a
thicknesses of about 150 nm.
[0017] Silicon oxide film 3 reduces a change of characteristics,
such as a frequency drift, of piezoelectric substrate 1 caused by a
change of temperature, such as a thermal expansion of the
substrate. Silicon oxide film 3 prevents interdigital electrodes 2
and 22 from short-circuiting. I order to reduce the change of the
characteristics of piezoelectric substrate 1 due to a change in
temperature, a film thickness H of silicon oxide film 3 and a wave
length .lamda. of the elastic wave satisfy the relation of
H/.lamda..gtoreq.0.15. The film thickness H of silicon oxide film 3
is a thickness from upper surfaces 2A and 22A of interdigital
electrodes 2 and 22, namely, a distance from upper surfaces 2A and
22A of interdigital electrodes 2 and 22 to upper surface 3A of
silicon oxide film 3. According to this embodiment, in the case
that the wave length of the elastic wave is 2 .mu.m, the film
thickness H of silicon oxide film 3 is determined to be 400 nm
which is larger than 300 nm which corresponds to the relation of
H/.lamda.=0.15. If upper surface 3A of silicon oxide film 3 is
rough, the thickness is a distance from upper surfaces 2A and 22A
of interdigital electrodes 2 and 22 to upper surface 3A of silicon
oxide film 3 directly above upper surfaces 2A and 22A. The upper
limit of the ratio H/.lamda. is determined to allow elastic wave
element 1001 to have an ordinary size as a product. For example,
the ratio is determined based on the size of a case accommodating
elastic wave element 1001 therein or on the size of a portion
having elastic wave element 1001 mounted thereto.
[0018] Elastic wave element 1001 excluding silicon nitride oxide
film 4 was produced as a comparative example of the elastic wave
element. The comparative example was enclosed in an airtight
container and left at temperature of 85.degree. C. for 1000 hours
as a high-temperature test. The center frequency of the comparative
example changed by about -1000 ppm.
[0019] Silicon oxide film 3 of the comparative example of the
elastic wave element was formed by a sputtering method using argon
gas and silicon oxide substrate as a target material. Silicon oxide
film 3 formed by this method has argon contained therein. When an
environment, such as a temperature, is changed, the contained argon
is discharged from silicon oxide film 3 to outside. This changes
physical characteristics, such as an elastic modulus and a density,
of silicon oxide film 3 caused a change of characteristics, the
change of the center frequency, of the comparative example of the
elastic wave element.
[0020] In elastic wave element 1001 according to the embodiment,
silicon nitride oxide film 4 is formed on an upper surface 3A of
silicon oxide film 3. Silicon nitride oxide film 4 is formed by a
sputtering method using a silicon oxide plate as a target material
and mixture gas of argon gas nitrogen gas as to vapor-deposit
silicon nitride oxide on upper surface 3A of silicon oxide film 3.
Silicon nitride oxide film 4 prevents argon contained in silicon
oxide 3 from being discharged from silicon oxide film 3, and
reduces the change of characteristics of elastic wave element 1001
according to temperature, thus providing elastic wave element 1001
with high reliability.
[0021] The comparative example of the elastic wave element and
elastic wave element 1001 according to the embodiment was subject
to an aging process to leave the elements at a temperature raised
further by 300.degree. C. for 12 hours. FIG. 2 shows a composition
rate of argon contained in silicon oxide film 3 of the elastic wave
elements. The composition rate of argon in silicon oxide film 3 of
the comparative example before the aging process was about 1.3%,
while the composition rate after the aging was less than 0.1%,
which shows much argon was discharged.
[0022] In the comparative example of the elastic wave element,
argon at upper surface 3A and vicinity of silicon oxide film 3 was
discharged from silicon oxide film 3 during the aging process to
produce depletion in the film. Argon near the depletion moves into
the depletion and then the moving argon was discharged. These
processes were repetitively executed and change physical
characteristics, such as the elastic modulus and the density, of
silicon oxide film 3, accordingly changing characteristics of the
elastic wave element.
[0023] FIG. 2 shows a change of composition rates of argon
contained in silicon nitride-oxide film 4 and silicon oxide film 3
of elastic wave element 1001 according to the embodiment. The
composition rate of argon in silicon nitride oxide film 4 before
and after the aging process was both about 0.4%, substantially no
difference, which shows no argon was discharged during the aging
process. The composition rate of argon in silicon oxide film 3 of
the elastic wave element according to the embodiment before and
after the aging process was both about 1.3%, substantially no
difference. Thus, the elastic wave element reduced the change of
the composition rate of argon in silicon oxide film 3 of elastic
wave element 1001 due to the temperature, and reduced the change of
the physical characteristics, such as the elastic modulus and the
density, of silicon oxide film 3 due to the change in the
temperature.
[0024] The elastic wave element including silicon nitride oxide
film 4 was enclosed in an airtight container and was left at a
temperature of 85.degree. C. for 1000 hours for a high-temperature
test. The change of the center frequency of the element was reduced
to less than about -250 ppm.
[0025] In the comparative example of the elastic wave element,
argon contained in silicon oxide film 3 is discharged during the
aging process, and produces depletion in silicon oxide film 3 along
a path through which the argon is discharged. The depletion allows
humidity to enter easily via the path, and deteriorates resistance
to humidity of the comparative example of the elastic wave element.
Upon being sealed in an airtight package, the comparative example
of the elastic wave element has its resistance to humidity
improved, but such airtight package is expensive. Elastic wave
element 1001 according to the embodiment prevents the contained
argon from being discharged from silicon oxide film 3, and prevents
silicon oxide film 3 from having therein the depletion, the path
through which humidity enter. This allows elastic wave element 1001
according to the embodiment to be sealed with less expensive resin
and to be manufactured at high productivity.
[0026] In the case that silicon oxide film 3 provided on
interdigital electrodes 2 and 22 is thick, the elastic wave
propagates not only on upper surface 1A of piezoelectric substrate
1 but also in a region including silicon oxide film 3. The
comparative example of the elastic wave element has the depletion
produced in silicon oxide film 3. The depletion causes a
propagation loss of the elastic wave, and increases an insertion
loss of the elastic wave element. However, the elastic wave element
according to the embodiment prevents silicon oxide film 3 from
having such depletion therein, accordingly reducing such insertion
loss.
[0027] In an elastic boundary wave element in which an elastic wave
propagates through a boundary between piezoelectric substrate 1 and
an insulating film on upper surface 1A of piezoelectric substrate
1, a propagation loss in the insulating film easily affects the
insertion loss of the elastic boundary wave element. Therefore,
silicon nitride oxide film 4 according to the embodiment is
preferably used in the elastic boundary wave element.
[0028] As described, silicon nitride oxide film 4 causing argon to
stay in silicon oxide film 3 maintains a frequency stability of
elastic wave element 1001 for a long time, and improves the
resistance to humidity and electrical characteristics of the
element.
[0029] Silicon nitride oxide film 4 has preferably a large
thickness as to prevent argon from being discharged from silicon
oxide film 3. A stress produced when silicon nitride-oxide film 4
is formed is greater than a stress produced when silicon oxide film
3 is formed. When an excessively large stress is produced on
silicon nitride oxide film 4, the stress transmits through silicon
oxide film 3 to piezoelectric substrate 1, and deteriorates
intermodulation (IM) characteristics of interdigital electrodes 2
and 22.
[0030] According to an experiment to check deterioration of the IM
characteristics executed by changing a ratio of the film
thicknesses of silicon nitride-oxide film 4 and silicon oxide film
3, it was confirmed that the IM characteristics deteriorates much
if the thickness of silicon nitride-oxide film 4 exceeds 3% of that
of silicon oxide film 3.
[0031] In order to prevent argon from being discharged from silicon
nitride oxide film 4, silicon nitride-oxide film needs to have a
thickness not smaller than 3 nm. Silicon nitride oxide film 4
having a thickness less than 3 nm may not prevent the argon from
being discharged. Namely, silicon nitride oxide film 4 deposited on
silicon oxide film 3 has preferably a thickness not smaller than 3
nm and not larger than 3% of the thickness of silicon oxide film
3.
[0032] Elastic wave element 1001 has a relatively simple structure
including interdigital electrodes 2 and 22 provided on
piezoelectric substrate 1. The elastic wave element according to
the embodiment provides various elastic wave devices, such as
filters including plural interdigital electrodes 2 and 22, a
diplexer, and a duplexer including these filters. The elastic wave
element according to the embodiment is particularly useful to a
transmission filter exhibiting the IM characteristics deteriorating
due to large power and a duplexer including the transmission
filter.
[0033] According to the embodiment, terms, such as "upper surface"
and "directly above", suggesting directions indicates relative
directions depending on relative positions of components, such as
piezoelectric substrate 1, interdigital electrodes 2 and 22,
silicon oxide film 3, and silicon nitride-oxide film 4, and not
indicates absolute directions, such as a vertical direction.
INDUSTRIAL APPLICABILITY
[0034] An elastic wave element according to the present invention
reduces fluctuation of propagation characteristics of elastic
waves, and has high reliability, thus being useful for a high
frequency circuit in communications equipment.
* * * * *